1. Field of the invention
The present invention relates to electrostatic charging devices, particularly those utilizing corona in a gaseous medium to induce charge on and in a receptor material. The present invention further relates, more particularly, to electrostatic charging devices which utilize a flowing fluid medium to convectively transport charge from an ionizing corona to a receptor surface.
2. Description of the Prior Art
A. Electret Theory
It has been known for a long time that polymer materials may be static electrically charged, or for brevity, charged. When charged, such polymers are known as "electrets". Electrets have significant commercial value. For instance, the electric field produced by the electret can be used to attract other materials, such as dust particles. This attractive or "inductive" property exhibited by electrets enables filters to be constructed having the ability to capture sub-micron particles when the filter media contains electret materials. Other examples of the value of electrets include their energy retention capability which may be utilized to provide a battery or used effectively in electrophotography.
As can be understood with reference to FIG. 1, an electret 10 may exhibit static electrical charge by any of several different mechanisms, most notably: selectively aligned molecular dipoles 12, injected space charges 14 and deposited surface charges 16. The charging process, itself, is accomplished by either a transfer of electrons to or from the material, thereby resulting in a net positive or negative charge, or an interior re-alignment (that is, polarization) of the protons and electrons on the molecular level, thereby resulting in a net charge as measured between different locations on the surface of the material (the total surface net charge caused thereby remaining zero), or a combination of each of the foregoing processes.
FIG. 2 exemplifies the standard commercial technique for production of electrets from roll mill polymer film stock. A high voltage (kilovoltage) power supply 18 is connected to an electrode 20. The electrode must have a sharp point, edge, corner or other similar feature becuase a location of small radius of curvature is known to produce a highest possible electric field in the shortest possible space. As a result of D.C. electrification of the electrode, the surrounding gaseous medium 22 (usually being composed simply of air) in the vicinity of the electrode 20 becomes ionized. The region defined by this ionized gaseous medium is known as corona 24. The corona extends downwardly from the electrode 20 toward a grounded base plate 26. For the most part, the gaseous medium 22 and the corona 24 are stable and not in motion. The exact size and shape of the corona depends upon many factors including: the voltage difference between the electrode and the grounded plate, the distance of their mutual separation, and their relative geometries, as well as the dielectric properties of the gaseous medium (as may be affected, too, by temperature and humidity).
In operation, the roll mill polymer 28 is fed through the corona 24, with the expectation that the corona will induce charge in the polymer by induction (resulting in the production of interior dipoles) and by conduction (resulting in charge being deposited on the surface). However, as can be seen from the middle depiction in FIG. 2, actually, when the roll mill polymer 28 enters the region between the electrode 20 and the grounded base plate 26, the nature of the dielectric space therebetween has been radically changed, resulting in the disappearance of the corona. Consequently, charging to the roll mill polymer is actually produced by induction between the electrode and the grounded base plate, without contribution from the ionization of the gaseous medium above roll mill polymer. The bottom-line is that the ultimate charge production in the roll mill polymer is compromised by the disappearance of the corona, so that the resulting electret 30 so produced, as shown in the bottom depiction in FIG. 2, is charged considerably below that level which is theoretically possible for the particular electret material.
Other methods of producing electrets are known and utilized with varying degrees of success.
Thermal charging methods heat a polymer sheet, causing reduction in the internal viscous forces binding the molecules and/or atoms which are arranged in a matrix or array. An external electric field is applied, thereby causing internal dipole production as molecules and/or atoms align with respect to the external electric field. The polymer sheet is then cooled and the external electric field is thereupon removed. Removal of the external electric field results in a "thermoelectret", as the aligned molecules and/or atoms are delayed for an extended time period from returning to their originally unaligned orientations due to viscous forces. This method is suitable only for dipolar polymers, and the considerable charging time required is a significant drawback.
Photoelectric charging methods utilize those polymers which exhibit photoconductivity. Light of a discrete quanta is directed at the polymer surface, imparting energy to the surface electrons. Under a process known as the photoelectric effect, electrons are ejected from the polymer. This method is generally not usable commercially, but has found some use in electrophotocopy technology for reversing electret charge.
Radio charging methods utilize a radio wave as an excitation medium to cause electrons to occupy temporarily higher energy states in otherwise forbidden energy bands. This movement of electronic charge creates a space charge within the polymer. This method is quite limited in applicability and the radio energy necessary is considerable.
Low-energy electron beam methods utilize an ion beam to irradiate the polymer surface. This method is plagued by difficulty in assuring uniformity of energy dispersion across the polymer surface. However, the mono-energetic electrons of these beams can be precisely controlled so as to achieve charge deposition to a desired predetermined depth. Accordingly, this method has gained widespread acceptance for producing electret diaphragms in electro-acoustic transducers.
Finally, contact (or triboelectric) charging methods utilize two dissimilar materials that are physically rubbed together. As a polymer and another, dissimilar, material are rubbed together, friction is the driving force that produces a net charge transfer across the interface between the materials. However, because of lack of reproducibility in the ultimate charge attained each time this process is performed, this type of charging method has found little acceptance in industry.
B. Examples of Prior Art Corona Chargers
Now, in the prior art there are various electrostatic charging devices that have been constructed which utilize corona charging. With due regard to the hereinabove recounted difficulties encountered with corona charging, the following patents offer various solutions.
U.S. Pat. No. 3,566,110 to Gillespie et. al, dated Feb. 23, 1971 discloses an electrostatic charging apparatus which is structured for use in electrostatic printing. The device utilizes a conventional corona charger upstream of a convective corona charger. The convective corona charger is composed of a conduit into which is located a charger device composed of: 1) a series of charger electrodes having a first polarity and located remote from the receptor surface and 2) a screen-like charger electrode having a second polarity and located adjacent the series of charger electrodes. A blower directs air past the charger device, the air becomes ionized, then convectively makes contact with the receptor surface.
U.S. Pat. No. 3,754,117 to Walter, dated Aug. 21, 1973 discloses a device for charging a layer of material utilizing a corona charger. An adjacent nozzle supplies a gas utilized to provide improved surface treatment resulting from the corona effect.
U.S. Pat. No. 4,153,836 to Simm, dated May 8, 1979 discloses a device for recording half-tone images in a photocopier device. A container is filled with nitrogen that is introduced through a conduit. Within the container is a corona discharge electrode. The nitrogen exits at a gap in a slotted diaphragm. The charge transfer characteristic is altered by varying voltage applied to two separated plates located at either side of the diaphragm.
U.S. Pat. No. 4,275,301 to Rueggeberg, dated June 23, 1981 discloses a device for deglossing a vinyl floor tile by utilization of corona discharge characteristic of a selected gas. The selected gas enters an upper plenum, travels to a lower plenum and exits the device on either side of a corona discharge electrode. Corona discharge exists in the gap formed between the corona discharge electrode and a ground electrode, the vinyl floor tile traversing the space therebetween.
U.S. Pat. No. 4,762,997 to Bergen, dated Aug. 9, 1988 discloses a fluid transport electrostatic charger used in electrostatic printing (photocopying). Air enters a plenum, then passes through a metering slit into a chamber housing a charger electrode. The air becomes ionized, then exits the charger so as to transfer charge to a receptor surface.
U.S. Pat. No. 4,745,282 to Tagawa et al, dated May 17, 1988 discloses a ventilated corona charger used in electrostatic printing. Ventilation is provided because of charge non-uniformity caused by irregularities in the atmosphere in and about the corona. A blower is supplied which directs a controlled stream of fresh air past electrode wires, thereby serving to stabilize the corona discharge characteristics.
U.S. Pat. No. 4,853,005 to Jaisinghani et al, dated Aug. 1, 1989 discloses an electrically stimulated filter, in which a perforated plate serves as one electrode and a series of parallel wires serve as the second electrode. A corona is established therebetween which charges in-coming air in advance of encountering an electrostatic filter device.
C. Discussion of the Prior Art
I have exhaustively studied the characteristics of corona discharge, and have found that the greatest difficulty in corona discharge has to do with maintenance of the corona when the receptor is being charged. This is due to variation in the dielectric value between the corona electrode and a grounded base as the receptor passes therebetween. I have determined that the only effective way to eliminate this problem is to engineer a charger in which the corona is not substantially affected by the presence of the receptor. My research has led me to the conclusion that this goal may be accomplished by creating a corona in a flowing gaseous fluid, the ionized fluid then contacting the receptor, thereby transferring charge at its surface.
Each of the patents cited above contemplate ionized gaseous fluids attendant to a charging process. Indeed, the patents to Simm, Bergen, Gillespie et al, and Tagawa et al contemplate specifically charging a sheet receptor by ionized gas convention between the corona electrode and the receptor. However, my research, as will be elaborated hereinbelow, indicates that these prior art devices do not effectively solve the problems associated with corona chargers used in the production of electrets. Simm, Bergen, Gillespie et al and Tagawa et al reference use of their respective devices in electrostatic copying machines. Electrostatic copiers impart only that minimum charge to the receptor which is necessary to effect printing. For comparison, this same charge exposure applied to a polymer receptor will only produce an inferior quality electret. What is needed in the art is an apparatus and method to achieve a maximum possible charge on the electret, a charge orders of magnitude greater than that used in electrostatic copying.
In order to maximize electret charge, an optimal charger is needed: one where charge is imparted on the receptor by use of ionization of a gaseous fluid convecting through a corona, so that the corona will not be diminished by the presence of the receptor; and where corona is maximized, geometry is optimized, and efficiency is able to be maintained for extended periods of operational time.
Referring once again to the above cited patents, several significant distinctions can be drawn to show that none of these offer a structure that serves as the optimal charger for production of electrets.
Gillespie uses a wire screen as an electrode; this is subject to quick clogging by dust particles. Further, Gillespie locates the electrodes far too remote from the receptor; the geometry is not optimum. Charge delivery is orders of magnitude below that which is required to produce quality electrets.
Walter has no sharp electrode edges; the corona is very weak.
Simm uses only a single needle point to provide an electrode and the needle point is positioned so that the nitrogen may easily by-pass the vicinity of the needle and never experience corona; the geometry is not optimum and corona is very weak.
Rueggeberg uses a very large electrode surface which is subject to quick contamination. Further, the electrode has no sharp edges, so it provides only weak corona.
Tagawa et al uses an electrode system composed of a plate with adjacent wire or wires; the plate is subject to rapid contamination. The geometry is not optimized and the electrode system will produce weak charging.
Bergen uses an electrode system composed of a wire in a cylinder; the cylinder is subject to rapid contamination. The electrode system is remote from the receptor; geometry is not optimized.
Jaisinghani et al uses a perforated metal plate as one electrode which is subject to quick degradation by contamination build-up. Further, air flow is restricted because the perforated plate is oriented transverse to the air flow stream.
Accordingly, what remains in the prior art is to provide an optimally configured charger using a convecting fluid in which the corona is optimized everywhere in the cross-section of flow of the convecting fluid.
These, and additional objects, advantages, features and benefits of the present invention will become apparent from the following specification.